ArcGeology is a highly normalized relational database implemented in an ArcGIS geodatabase. Consequently, the lithologic and temporal data, which have many-to-many relationships to the map units, is not neatly available for symbolization in the format that ArcMap requires. Via a fuzzy mathematical process, "computing with words," the expert system allows combination of multiple rock types and/or times into a composite, interpretive property for each map unit. Thus, the expert system facilitates construction of derivative maps symbolized with environmental properties such as erosion potential, macronutrient availability, acid buffering, ground shaking potential, transportation suitability, etc., which can be interpreted from lithology, time, and other geologic and topographic data.